The present invention relates generally to non-volatile semiconductor memory devices, and more particularly, to a system for reading the bits in a double-bit memory cell and associated reference cells.
For conventional non-volatile semiconductor devices, such as flash memories, each memory cell stores only one bit of data. For a conventional single bit per cell flash memory architecture, each cell typically includes a metal oxide semiconductor transistor structure having a source, drain, and a channel in a substrate, and a stacked gate structure over a channel. The stacked gate structure typically includes a tunnel oxide formed on the surface of the channel, a floating gate on the tunnel oxide layer, a thin oxide layer over the floating gate, and a control gate overlying the oxide layer. Information is programmed into a flash memory by charging the floating gate for each individual core cell to a predetermined voltage threshold. For a single-bit memory cell, two threshold voltages are predefined such that the data bit is programmed at a threshold that is either a binary xe2x80x9c0xe2x80x9d or a binary xe2x80x9c1xe2x80x9d.
Modem devices having non-volatile memory are placing ever increasing demands for larger memory in a small profile package. To satisfy this demand for higher density memory, double-bit per cell (xe2x80x9cdouble-bit cellxe2x80x9d) flash memories have been developed. The double-bit cell refers to a cell that allows the storage of two bits of data into a single memory cell.
FIG. 1 is a schematic diagram of the double-bit core cell structure for the present invention. For each double-bit core cell 10, two bits can be stored and programmed, a first bit identified as the normal bit and a second bit identified as the complementary (xe2x80x9ccompxe2x80x9d) bit. For a non-volatile memory such as flash memory, the integrity of the data must be maintained over the life of the device to avoid data errors that could adversely impact a user. Over the life of a flash memory cell there can be changes in the cell characteristics which can affect the data. One such change for a cell can occur due to the programming of an adjacent cell over time which disturbs the state of nearby cells. Charge loss over the life of the flash memory can also affect the cell characteristics. These changes in the cell characteristics can result in a shifting of the cell threshold voltages. This shifting can affect the state of the memory cells so as to create data errors. As a result, techniques are needed to assure the integrity of the data bits in a cell over time.
FIG. 2a shows two waveforms illustrating an example of the shifting in the distribution of the thresholds for a double-bit cell over time. Since there are two bits in the cell, four threshold distributions are used for the combinations of the two bits. Trace A shows an exemplary state of the voltage threshold distribution for a new memory cell (referred to in FIG. 1 as xe2x80x9cbefore lifexe2x80x9d) that has not undergone any programming or read cycling. As memory cells in the array are cycled over the life of the cell array, charge loss, memory disturb, and other changes occur over time that can affect the device characteristics. The changing device characteristics can shift the threshold distribution into a different state. An example of the shifted state (referred to in FIG. 1 as xe2x80x9cafter lifexe2x80x9d) is shown in Trace B. For example, in the traces in FIG. 2a, a (1,0) represents a normal bit equal to 1, and a second xe2x80x9ccomplementaryxe2x80x9d bit equal to 0. Non-volatile memories such as flash memories might be programmed only once and then read intermittently over a long period of time. There is thus a need to determine the data bit values for the double-bit cell accurately for both threshold distributions shown in Trace A and Trace B. In one technique, two reference thresholds (REF1 and REF2) are provided and compared to the core cell threshold in order to determine the cell data. FIG. 2b illustrates how the technique determines the core cell data from the comparison of the reference thresholds to the normal and complementary bits.
As shown in FIG. 2a and the top and bottom data rows in FIG. 2b, if the core cell threshold lies at either of the two extremes of the distribution, (1,1) or (0,0), only a comparison of the normal bit to the two reference thresholds is required in order to determine the data. For this case, since a comparison with the complementary bit is not needed for determining the cell data, the xe2x80x9ccomp bitxe2x80x9d column in FIG. 2b is marked as an xe2x80x9cxxe2x80x9d representing a xe2x80x9cdon""t carexe2x80x9d state. For example, the core cell data should be xe2x80x9c0xe2x80x9d when the core cell threshold voltage is higher (identified as xe2x80x9c0xe2x80x9d) than the two reference thresholds, as shown in the bottom row in FIG. 2b. Conversely, for a core cell threshold voltage lower than the REF1 and REF2 thresholds, the data should be xe2x80x9c1xe2x80x9d.
The shifting of the distribution thresholds over time from Trace A to Trace B, as illustrated by example in FIG. 2a, presents challenges in determining the data for various core cell thresholds. When a core cell threshold lies in the area between the two reference thresholds, the data cannot be determined solely by comparing the normal bit with the two reference thresholds. This is illustrated by FIG. 2a and FIG. 2b, where for a normal bit of 1 for (1,0) in Trace A, and a normal bit of 0 in (0,1) in Trace B, the comparison to the two reference threshold voltages, REF1 and REF2, yields the same result. In order to attempt to provide for proper determination of the data in the two cases, a technique has been developed that provides for two sequential data reads along with reading the two reference thresholds from two reference cells. If both the normal bit and complementary bit can be read along with the reference cells, then the cell data for either the trace A (before life) or Trace B (after life) distribution can be determined, as explained in more detail below.
The advantage of having the complementary bit in addition to the normal bit is illustrated in FIG. 2b. For this technique, when the core cell threshold lies between two reference thresholds, identified as REF1 and REF2, both the normal bit and the complementary bit are compared to REF1 and REF2. For example, for the case above of a normal bit of 0 in (0,1) in Trace B, for this technique the complementary bit threshold lies under the (1,0) distribution, the opposite of the (0,1) distribution. Thus for a core cell threshold between REF1 and REF2, the complementary bit threshold would not be between the two thresholds and thus can be determined. By illustration, the core cell threshold lies in the area under the (0,1) distribution in Trace B between REF1 and REF2. This case is shown in the next to last row of FIG. 2b. For the normal bit, the core cell threshold is higher than REF1 (shown as xe2x80x9c0xe2x80x9d in the table) and lower than REF2 (xe2x80x9c1xe2x80x9d). For Trace B, the complimentary bit corresponding to this core cell threshold lies under the (1,0) threshold. This complementary bit threshold is lower (xe2x80x9c1xe2x80x9d) than both REF1 and REF2. This results in a cell data of 0 as shown in FIG. 2b. 
Similarly, as shown in the third row from the bottom of FIG. 2b, the comparison is the same for the normal bit and REF1 and REF2 for the area between REF1 and REF2 under the (1,0) distribution in Trace A. Using the complementary bit results in determining the cell data for that case to be a xe2x80x9c1xe2x80x9d.
For the above technique, by adding a additional comparison of the complementary bit to the reference cells, REF1 and REF 2, the cell data can be accurately determined over the life of the cell, even for the case when the core cell threshold lies between the two reference cell thresholds. For the above method, each bit (normal and complementary) is read along with the two reference cells for the reference thresholds REF1 and REF2. The combination of the data read for the reference cells determines the actual cell data. The system and method to provide for the reading of the complementary bit in order to provide the required comparison, is described in further detail below.
FIG. 3 shows the circuit diagram for the sense circuit architecture corresponding to the method described above. As can be seen from FIG. 3, a sense circuit 20 includes a data circuit 30 coupled to a sense amp 22, and a reference circuit 50 coupled to a reference sense amp 24. The outputs of the sense amp 22 and sense amp 24, on lines (xe2x80x9cSAxe2x80x9d) and (xe2x80x9cSARxe2x80x9d) respectively are coupled to the input of a comparator 26. Comparator 26 is provided for comparing the output signal SA from the data sense amp 22 with the output signal SAR from the reference sense amp 24 in order to determine the data.
Data circuit 30 includes a core cell 10 having a control gate connected to the word line, a drain connected to a node 35 and a source connected to a node 33. The memory cells in a cell array are typically organized by row and column. The common word line is provided by a control circuit (not shown) for selecting a row for the cell in the array. A VCC signal is also provided by the control circuit for selecting the column of the memory cell to be accessed. The details of the control circuit for the decoding and addressing of an individual cell in a memory array are well known to one of ordinary skill in the art. The present invention is described in further detail for a single core cell 10.
For the reading of the complementary bit, the drain and source of the core cell 10 must be swapped, as compared to the reading of the normal bit. Thus there is one path to the DATA bit line provided for the reading of the normal bit and a different path provided for reading the complementary bit. As shown in FIG. 3, additional transistors are typically provided as pass transistors for providing the two conduction paths to the DATA bit line, as will be described in more detail below.
In the data circuit 30 in FIG. 3, the source terminal of the core cell 10 connects, at node 33, to the drain terminal of a pass transistor 32. Pass transistor 32 has a gate connected to VCC and a source connected to a node 31. A transistor 38 has a drain connected to node 31, a source connected to the input of sense amp 22 at node 37, and a gate connected to a xe2x80x9c2ndxe2x80x9d line. This 2nd line is provided by a control circuit (not shown) and provides control for the selection of the path for the reading of the second of the two bits in the double-bit cell (the complementary bit). Thus, if the second bit of core cell 10 is to be read, the gate inputs of the transistors in data circuit 30 will be controlled so as to provide for a connection path from the source terminal of core cell 10 at node 33, through transistors 32 and 38, to node 37 which connects to the DATA bit line at the input to the data sense amp 22. Node 31 is also connected to the drain of a transistor 44. The source of transistor 44 is connected to ground and the gate is connected to a xe2x80x9c2nd bar linexe2x80x9d as shown in FIG. 3. Transistor 44 provides a path to ground that is needed for the reading of the normal (first) bit.
Node 35, at the drain terminal of core cell 10, connects to a source terminal of a pass transistor 34 having a gate connected to VCC and a drain connected to the source of a transistor 36 at node 39. Transistor 36 has a drain connected to the DATA bit line at the input of sense amp 22 at node 37. The gate for transistor 36 is connected to a xe2x80x9c1stxe2x80x9d line. This 1st line provides control for reading the first of the two bits (the normal bit). Node 39 is also connected to the drain of a transistor 42. The source of transistor 42 is connected to ground and the gate is connected to a xe2x80x9c1st bar linexe2x80x9d. Transistor 42 provides a path to ground for the drain of cell 10 for the reading of the complementary (second) bit. The 1stand xe2x80x9c2nd barxe2x80x9d signal are thus active for the reading of the normal bit, and the 2nd and xe2x80x9c1st barxe2x80x9d are active for the reading of the complementary bit. The VCC and word line signals as described above, are set active when core cell 10 is selected.
Reference circuit 50 will now be described with reference to FIG. 3. Although only one reference circuit 50 (e.g. for REF1) and sense amp 24 and comparator 26 is shown if FIG. 3, sense circuit 20 would include an identical circuit (not shown) associated with the second reference cell for the second reference threshold, REF2. Reference circuit 50 includes a reference cell 52 having a control gate connected to the reference word line, a source connected to ground, and a drain connected to a node 55. Two transistors 54, 56 are connected in series between node 55 and the input of the sense amp 24. The gates of transistor 54, 56 are shown connected to VCC. The source of transistor 54 is connected to node 55 at the drain terminal of reference cell 52. The drain of transistor 54 and source of transistor 56 are both connected at node 57. The drain of transistor 56 connects to the input of the sense amp 24 as shown at the reference signal DATAR in FIG. 3.
The operation of the sense circuit 20 will now be described with reference to FIGS. 3 and 4. For the method corresponding to FIG. 3, each bit (normal and complementary) is read along with the two reference cells. The comparisons of the voltages for the bit and reference cells for the two reads determines the actual core cell data. For the circuit in FIG. 3, the normal bit and complementary bit cannot be read at the same time, because the reading of the second (complementary) bit requires the swapping of the source and drain of core cell 10. In order to obtain both data bits in the double-bit cell for circuit 20 there must be two separate reads: one read for the normal bit (and reference cells), and a second read for the complementary bit (and reference cells). Because of the need for two reads, the access time required to complete both reads is critical in determining how fast both bits are read accurately.
A key determining factor in the total read access time is how fast the second (complementary bit) read is completed after the first (normal bit) read. A drawback of the sense circuit 20 and method in FIG. 3, is the bit line undershoot that delays the execution of the second read. FIG. 4 illustrates this undershoot drawback. The voltage traces in FIG. 4 show the voltages over time as the first read is completed and the second read is executed. Trace C in FIG. 4 shows a waveform for the sense amp signal (SA) from FIG. 3 for the output of the data sense amp 22. Trace D is a waveform for the voltage for the corresponding data (DATA) at the bitline. Trace E is a waveform for the voltage for the SAR signal from the reference sense amp 24; and Trace F represents the reference cell data voltage (DATAR) input to the sense amp 24. In operation, when the second bit is to be read, the source and drain are swapped. As a result, the drain of core cell 10 starts to charge and the source begins to be discharged. This transition period causes a huge voltage drop in the bit line (DATA) which becomes a huge voltage drop xe2x80x9cundershootxe2x80x9d), in the corresponding sense signal SA in circuit 20 as shown in Trace C in FIG. 4.
As shown in FIG. 4, after this undershoot in SA occurs, there is a significant time interval before the SA signal can return to a desirable level in order to perform the second read. This time interval is the xe2x80x9crecovery timexe2x80x9d identified in FIG. 4. In operation, the SA signal must recover sufficiently so that there is the required margin between the SA and SAR signals in order for the second read to proceed. This large recovery time is a drawback of the circuit 20 of FIG. 3 since it results in a significant time loss in the total access time to complete the two required reads.
Modern devices increasingly are requiring faster memory access times in order to process the data at a rate that can provide the response that users demand. Access times for a flash memory must be optimized while increasing the density. There is therefore a need for providing optimized memory access times for a double-bit flash memory while maintaining the integrity of the memory data over the life of the memory.
Therefore, it would be desirable to have a system and method for reducing the recovery time for the second read in order to provide faster access time for execution of the two reads for a double-bit memory cell in a nonvolatile memory.
The present invention provides a system and method for reference cell swapping for reducing the recovery time for the second read for a double-bit cell in a non-volatile memory, such as a flash memory. The reduction in recovery time reduces the overall read time for the two reads substantially compared to other techniques. For the present invention, two consecutive reads are performed with each bit in the double-bit cell being read along with two reference cells. The system includes a circuit to provide for the swapping of the source and drain terminals of both the reference circuit and the data circuit in order to efficiently and correctly read the data. The system of the present invention includes a circuit and method that accounts for changes in the memory cell characteristics over time and thereby eliminates data errors.
In an embodiment of the present invention, an apparatus for providing for reading two data bits and two reference cells. The threshold voltages of each data bit and two reference cells are sensed. The two reference cell threshold voltages are each separately compared to the data bit sensed voltage threshold. A second complementary bit of the double-bit cell is read after a normal bit by swapping the core cell drain and source terminals. In order to substantially reduce recovery time for the second read, the reference cell drain and source are also swapped for the second read such that the sense reference threshold voltage has a drop (undershoot) that occurs due to the swapping, and that this undershoot tracks with a corresponding undershoot for the sensed data threshold voltage. The system of the present invention thereby reduces recovery time for the second read by enabling the sense signal for the data and sense signal for the reference cells to track each other. The tracking of the voltage undershoots that is provided permits the sense data voltage signal to reach the desired level in order to enable the second read to proceed and be executed much faster than for the known systems and methods.
Thus, the system of the present invention achieves fast read access times since the two bits in a double-bit memory cells are read with minimized recovery time between reads while also obtaining the desired accuracy in reading the data values despite characteristic changes of the memory cell over time.